The present invention relates generally to the production of metallic aluminum from alumina (Al.sub.2 O.sub.3) and more particularly to a method and apparatus for electrolytically reducing alumina to aluminum.
For many decades, the principle commercial method employed for the electrolytic reduction of alumina to aluminum has been the Hall-Heroult process. This process employs a cell comprising a vessel or pot containing a molten electrolyte bath comprising sodium cryolite (Na.sub.3 AlF.sub.6) as the principal constituent. The interior of the vessel is lined with carbon. A pool of molten aluminum lies on the bottom of the vessel and forms the cathode for the cell, and consumable carbon anodes located above the electrolyte bath extend downwardly through the top of the electrolyte bath. Alumina is introduced into the molten electrolyte bath wherein the alumina dissolves and a number of reactions occur, eventually producing molten aluminum which accumulates at the bottom of the vessel and carbon dioxide, and some carbon monoxide from a side reaction, which are given off from the top of the cell.
There are a number of drawbacks and disadvantages to the Hall-Heroult process, and these are discussed in some detail in Beck, et al., U.S. Pat. No. 4,592,812, the disclosure of which is incorporated herein by reference. One of the drawbacks of the Hall-Heroult process is that it employs consumable carbon anodes which must be periodically vertically adjusted during the electrolytic reduction operation and which also must be frequently replaced when the anode has been consumed down to a butt.
Attempts have been made to develop non-consumable anodes for use in the Hall-Heroult process to replace consumable carbon anodes. The non-consumable anodes are typically composed of a nickel-iron-copper cermet (a mixture of oxide and metallic particles). Examples of this and other materials developed for use in non-consumable anodes are described in the following U.S. Pat. Nos.: Ray 4,374,050, Ray 4,399,008, Ray et al. 4,454,015 and Ray et al. 4,455,211; and the disclosures of these patents are incorporated herein by reference.
Non-consumable anodes of the type described above have been employed in conjunction with a cryolite electrolytic bath, similar to that employed in the Hall-Heroult process, having a conventional operating temperature of about 950.degree. C. (1742.degree. F.). Three basic problems have been encountered with these non-consumable anodes: corrosion of the anodes in the bath, bath penetration into the anodes and fracture of the anodes. These problems must be overcome before the non-consumable anodes can be employed in commercial aluminum reduction cells. Attempts have been made to overcome these problems by improving the properties of the non-consumable anode materials, but even with the improved properties thus far obtained, the anodes still fall short of the goal for operation at the conventional Hall-Heroult process temperature of 950.degree. C. (1742.degree. F.).
The above-noted Beck, et al., U.S. Pat. No. 4,592,812 discloses an alumina electrolytic reduction cell employing non-consumable cermet anodes operating in a preferred temperature range of 700.degree.-800.degree. C. (1292.degree.-1472.degree. F.). In this cell, the anode is located at the bottom of the cell, and the cathode is horizontally disposed above the anode. The electrolytic reaction in this cell generates oxygen at the anode rather than carbon dioxide as in the Hall-Heroult cell. The alumina tends to sink toward the bottom of the cell, because its density is greater than the density of the electrolytic bath, but the alumina is maintained in suspension within the bath adjacent the bottom of the cell by the rising oxygen bubbles generated at the anode. The alumina saturates the bath next to the bottom and retards the corrosion rate of the anode located there.
The density of the bath is greater than that of the aluminum and molten aluminum formed at the cathodes rises to the top of the bath. The cathodes are non-consumable and are composed of titanium diboride which is wet by aluminum which thus follows the surface of the cathode as it rises to the top of the bath. Refractory barriers at the top of the cell provide channels for the oxygen to escape the bath without contacting the aluminum pool accumulating at the top of the bath.
The electrolytic reduction cell described in the Beck, et al. patent eliminates many of the drawbacks and disadvantages of the Hall-Heroult cell, and this is discussed in detail in the Beck, et al. patent. Nevertheless, there are drawbacks to this arrangement and these include the need to employ a horizontal anode located on the bottom of the cell and horizontal cathodes.